A Mach-Zehnder modulator (hereinafter referred to as an “LN modulator”) produced using lithium niobate (LiNbO3) or the like is known as a modulator for an optical transmission system. The LN modulator has been widely used for a high-speed optical transmission system at 10 GHz or more owing to its excellent high-speed characteristics and chirp characteristics. Since not only the LN modulator but also other components are mounted on an optical transceiver equipped with the LN modulator, it is preferable to miniaturize the LN modulator in order to realize high-density mounting.
In the LN modulator, for example, an optical waveguide formed on a substrate and an input/output optical fiber coupled to the optical waveguide are arranged so as to extend in the same direction. Therefore, a space for arranging the optical fiber along the extending direction of the optical waveguide is created. Accordingly, the size of the LN modulator is increased along the extending direction of the optical waveguide.
In order to suppress such an increase in the size of the LN modulator, another LN modulator has been proposed in which an optical waveguide and an optical fiber are arranged so as to extend in different directions. In this LN modulator, an end surface of a substrate at an end of the optical waveguide is formed into an inclined surface, and light emitted from the optical fiber arranged along a direction crossing the optical waveguide is reflected by the inclined surface of the substrate and enters the optical waveguide.
Japanese Laid-open Patent Publication No. 2004-125854
Although a device is expected to be miniaturized by using a conventional structure in which light emitted from an optical fiber is reflected by an inclined surface of a substrate and enters an optical waveguide, there has been a problem that the optical waveguide and the optical fiber extending in different directions are not easily coupled to each other.
Specifically, in the conventional structure, since the light emitted from the optical fiber is reflected by the inclined surface of the substrate, work for adjusting a positional relation between the inclined surface of the substrate and the optical fiber is complicated. In the conventional structure, therefore, the optical waveguide and the optical fiber extending in the different directions are not easily coupled to each other.
In order to address this problem and omit the above-mentioned position adjustment of the optical fiber with respect to the inclined surface of the substrate, another LN modulator has been developed in which an inclined surface is not formed on a substrate, and light emitted from an optical fiber is reflected by a reflection member and enters an optical waveguide. In such an LN modulator, since the reflection member is arranged at a position apart from the substrate, an optical path from a distal end of the optical fiber to the optical waveguide is longer than that of a structure in which an end surface of a substrate at an end of an optical waveguide is formed into an inclined surface. In this case, a loss of light occurs due to the long optical path from the distal end of the optical fiber to the optical waveguide. A possible way to suppress the loss of light is to employ such a structure that a condenser lens such as a collimate lens is arranged between the reflection member and the optical waveguide. In this structure, light reflected by the reflection member enters the condenser lens, and is collected by the condenser lens into the end of the optical waveguide.
In the structure in which the condenser lens is arranged between the reflection member and the optical waveguide, however, a space for arranging the condenser lens as well as the reflection member is created. As a result, miniaturization of the device is inhibited.